I'd like to find some pictures converted for the STE with display in 4096 colours.The type of pictures where the 16 colours of the palette entries are redefined once on each scanlineI would like to know the encoding format of these pictures, and know which pictures viewers giving the best stats about these pictures (overall and per scanline) are.Also if there are some sources (C or ASM) for these pictures viewers, please point me to them.

Same question if there are formats with more than 1 palette redefinition per scanline, if it exists (maybe with some constraints, as I guess there must be a non fixed delay, giving a few pixels of imprecision, when the 2nd set of 16 colours is used on the same scanline).

I dealt with this. Most what could achieve is 80 colors per scanline, but that was with special cartridge adapter, and transfer speed of some 3.6 MB/sec.Normally, you can copy max about 2MB/sec with STE. That means about 50 colors per line. That would be 200x50=10000, so yes, can have all 4096 colors at once on screen.You should search here for dml's posts about his Photochrome format. There are diverse versions, and even custom formats are possible.Then, there is mode with 2 alternating screens, where palettes slightly differ, so visual perception is that there are more colors than 4096,I used some of them, and wrote myself displaying code in ASM - hardly possible in C, it must be cycle accurate. Now I using cyg's system - it is basically very similar, just no alternate mode (what of course takes 2x so much space on disk), but conversion tool is better for my needs - dithering is used for better visual impression. And it is possible to combine it with overscan, so multi color pictures with res of 416x273 for instance. I used them even in movie player.I can give you asm src. for displaying. But you need to add user interface, loader if you want to make some viewer.

Answer on you question is already done partially. The whole thing is in updating shifter palette registers in exact moment of scanline. So, delays must be actually very accurate - that's mentioned cycle accuracy.Not to forget Spectrum 512, which was first doing palette change "on fly". It has it's viewer PRG too.I really don't know is some other high-color picture viewer for Atari made. I have viewer/converter to BMP of Photochrome older format in Windows SW:http://atari.8bitchip.info/floimgd.php

Famous Schrodinger's cat hypothetical experiment says that cat is dead or alive until we open box and see condition of poor animal, which deserved better logic. Cat is always in some certain state - regardless from is observer able or not to see what the state is.